1. Field of the Invention
The present invention relates to an apparatus and a method for shortening a communication time between nodes in an optical network, and more particularly to an apparatus and a method for shortening a processing time at each node.
2. Description of the Related Art
Recently, demand for the very high speed Internet service and various multimedia services has increased dramatically. In order to efficiently cope with the increasing demand, it is necessary to provide a broadband communication network adapted to new services providing easy transmission of visual and image information as well as voice and data. For integrating and unifying the various services, a broadband integrated services digital network (B-ISDN) is provided. In addition, in order to easily transmit voice, data, and image information, a high frequency band is required, such as a millimeter wave band, while taking into consideration transmission capacity and radio range. To meet these requirements, an optical network is typically suggested. In the optical network, nodes are connected to each other through an optical cable, so that an optical signal is transmitted between the nodes.
Various transmission techniques are used in the optical network. An example of these transmission techniques is wavelength division multiplexing (hereinafter, referred to WDM). According to WDM, a low loss wavelength band of an optical fiber is divided into a plurality of narrow channel wavelength bands, and a channel wavelength band is assigned to each input channel, thereby simultaneously transmitting input channel signals through the assigned wavelength bands. Accordingly, in the WDM, channel wavelength bands are independent of one another regardless of transmission data format, so an analog signal and a digital signal can be simultaneously transmitted. In addition, it is possible to transmit signals having transmission rates that differ from each other. Since the WDM has superior extensibility and permeability, the WDM may be widely used in the future.
On the other hand, an apparatus called “optical cross-connect” (hereinafter, referred to OXC) is required in the optical network using the WDM for switching optical signals to a plurality of subscribers. The OXC cross-connects the optical signals upon receiving the request of an operator or for fault recovery. In addition, the OXC inspects information regarding a channel's state and quality of the optical signal by receiving an inspection control signal, and transmits information to other nodes. In particular, the inspection and the cross-connection functions of the OXC should be performed at a high speed for rapidly recovering from a fault. Further, the OXC is required to have a low input loss, a low optical interference, and a low price.
The OXC are divided into electro-optic OXC and opaque OXC depending on transparency thereof. The opaque OXC is superior to the electro-optic OXC in view of the price and a wavelength variation. However, the opaque OXC is not transparent, so an optical transmitter and an optical receiver, which automatically detect a transmission speed of a transmission signal so as to convert the transmission signal, are used in the opaque OXC.
FIG. 1 is a schematic view illustrating a line switch caused by a fault in a predetermined section of an optical network using a conventional opaque OXC. In FIG. 1, a node B 100 is a transmission side, a node C 101 is a receiving side, and the fault occurs at a route 102 between the node B 100 and the node C 101. Nodes shown in FIG. 2 stand for the opaque OXC. In the description that follows a process for automatically converting the transmission signal by using the optical transmitter or the optical receiver is called “stabilizing process”.
Referring to FIG. 1, each node transmits and receives a signal by using a predetermined wavelength formed between adjacent nodes. For example, the node B 100 transmits the signal to the node C 101 by using a wavelength (λ31), and receives the signal from the node C 101 by using a wavelength (λ21). In addition, the node B 100 transmits the signal to a node D 103 by using the wavelength (λ21), and receives the signal from the node D 103 by using the wavelength (λ21). At this time, since a transmission route is separated from a receiving route, there is no interference between the nodes even if the same wavelength is used. Accordingly, in a normal state, a transmission signal to be transmitted into the node C 101 is inputted in the node B 100, and the node B 100 transmits the transmission signal to the node C 101 by demodulating the transmission signal through the WDM by using the wavelength (λ31). Then, the node C 101 demodulates the signal and outputs a demodulated signal as a receiving signal. At the time, the transmission signal is inputted into one of channel cards forming the node B 100, and the receiving signal is outputted through one of channel cards forming the node C 101.
However, when a line switch is required due to a fault between the node B 100 and the node C 101, the node B 100 sets a new route by means of a predetermined routing table. As shown in FIG. 1, the new route 104 is connected to the node C 101 through a node D 103. At this time a new wavelength (λ32) is created between the node B 100 and the node D 103, and another new wavelength (λ33) is created between the node D 103 and the node C 101. The new wavelengths are selected so as not to create interference with existing wavelengths.
FIG. 2 is a flow diagram illustrating a signal processing flow in the normal state of the node B 100 and node C 101 illustrated in FIG. 1.
Referring to FIG. 2, the transmission signal 200 is inputted into a predetermined optical receiver provided in the node B 100. The predetermined optical receiver measures the transmission speed of the inputted transmission signal through an automatic transmission speed converting function and performs the stabilizing process with respect to the measured transmission speed. The stabilizing process prepares the environment required to process the signal based on the measured transmission speed. In FIG. 2, a time required for the optical receiver to perform the stabilizing process 201 is defined as “turnaround time A”. On the other hand, when the stabilizing process is finished, the optical transmitter converts the inputted transmission signal into an electric signal and transmits the electric signal to a switch 202. The switch transfers the electric signal to a predetermined optical transmitter according to the control of a main control unit (hereinafter, referred to MCU 203), which controls the whole operation of the node B 100. The switching 202 action of the electric signal is carried out at a high speed controlled by the MCU 203, so a time required for switching the electric signal is not illustrated in FIG. 2. The predetermined optical transmitter measures the transmission speed of the electric signal provided through the switching action, and performs the stabilizing process 204 with respect to the measured transmission speed. The stabilizing process prepares the environment required to process the signal based on the measured transmission speed. In FIG. 2, a time required for the optical transmitter to perform the stabilizing process is defined as “turnaround time B” 204. On the other hand, when the stabilizing process has finished, the optical transmitter converts the electric signal into the optical signal and transmits the optical signal to the node C 101. Accordingly, the total time spent at the node B 100 to transmit the optical signal to the node C after receiving the transmission signal is defined as “turnaround time A+turnaround time B”.
On the other hand, the optical signal transmitted from the node B 100 is received in a predetermined optical receiver of the node C 101. In FIG. 2, a time required for transmitting the optical signal from the node B 100 to the node C 101 is defined as “turnaround time C” 205. The node C 101 processes the optical signal in the same manner as the node B 100. Accordingly, the total time spent at the node C to process the optical signal and output the optical signal as the receiving signal 206 is identical to the total time spent at the node B 100. That is, the total time spent at the node C 101 is defined as “turnaround time A+turnaround time B”.
Therefore, in a normal state, the total turnaround time for transmitting the transmission signal from the node B 100 to the node C101, that is, the total stabilizing time required for normally transmitting the signal is defined as “2(A+B)+C”.
FIG. 3 is a flow diagram illustrating a signal processing flow when the line switching is required due to the fault between the node B 100 and the node C 101 shown in FIG. 1.
Referring to FIG. 3, when the fault occurs in a route of the node C 101, the node B 100 performs a detour route switching. As a detour route, a route 104 connected to the node C 101 via the node D 103 is used. Accordingly, the predetermined optical receiver provided in the node B 100 measures the transmission speed of the inputted transmission signal through an automatic transmission speed converting function and performs the stabilizing process with respect to the measured transmission speed. In FIG. 3, a time required for the optical receiver to perform the stabilizing process is defined as “turnaround time A” 300. On the other hand, when the stabilizing process finished, the optical transmitter converts the inputted transmission signal into an electric signal and transmits the electric signal to a switch 301. The electric signal switched by the switch is inputted into the optical transmitter corresponding to the node D 103 having a new route to be switched. At this time, the switching action of the switch is carried out under the control of the MCU 302. The optical transmitter measures the transmission speed of the electric signal provided through the switching 301 action and performs the stabilizing process with respect to the measured transmission speed. At this time, a new wavelength (λ22) for transmitting the signal to the node D 103 is determined through the stabilizing process. In FIG. 3, a time required for the optical transmitter to perform the stabilizing process is defined as “turnaround time B” 303. When the stabilizing process has finished, the optical transmitter converts the electric signal into the optical signal by using the wavelength (λ22) and transmits the optical signal to the node D 103.
The optical signal transmitted from the node B 100 is received in a predetermined optical receiver of the node D 103. At this time, although it is not explained, the line switching caused by the fault has already been sent to the node D 103 from the node B 100. Accordingly, the node D 103 decides, based on the predetermined routing table, to transmit the optical signal received from the node B 100 to the node C 101. At this time, the wavelength (λ33) for transmitting the signal to the node C 101 is determined. Then, the optical signal transmitted from the node B 100 is introduced into the predetermined optical receiver of the node D 103, and the optical receiver converts the optical signal into the electric signal through the stabilizing process. A time required for the stabilizing process is defined as “turnaround time A” 305. The converted electric signal is inputted into the switch 306 and switched to the predetermined optical transmitter by the switch 306 which 20 is controlled by the MCU 307 of the node D 103. The optical transmitter to be switched matches with the node C 101 corresponding to a final transmission terminal. The optical transmitter converts the electric signal into the optical signal through the stabilizing process and transmits the optical signal to the node B. At this time, a time required for the optical transmitter to perform the stabilizing process is defined as “turnaround time B” 308. In order to transmit the optical signal to the node C 101, the wavelength (λ33) is used.
The optical signal transmitted from the node D 103 is received in the node C 101. The node C 101 converts the optical signal into the electric signal through the stabilizing process 309 and transmits the electric signal to the switch 310. In order to perform the stabilizing process in the optical receiver, “turnaround time A” is needed, as required in other optical receivers for performing the stabilizing process. The electric signal converted by the optical receiver is transmitted into the optical transmitter through the switching 309 action by means of the switch which is controlled by the MCU 311 of the node C 101. The optical transmitter converts the electric signal into the optical signal through the predetermined stabilizing process 312 and outputs the optical signal as a receiving signal 313. At this time, in order to perform the stabilizing process in the optical transmitter, “turnaround time B” is needed, as required in other optical transmitters for performing the stabilizing process.
On the other hand, in FIG. 3, a time required for transmitting the optical signal from the node B 100 to the node D 103, and a time required for transmitting the optical signal from the node D 103 to the node C 101 are both defined as “turnaround time C”.
Accordingly, when the line switching occurs because of the fault, the total time for transmitting the transmission signal from the node B 100 to the node C 101, that is, the total stabilizing time, is defined as “3(A+B)+2C”.
As discussed above, the optical transmitter and the optical receiver used in the conventional opaque OXC automatically detect the transmission speed of the inputted signal in order to convert the transmission signal. For this reason, the conventional opaque OXC causes a time delay because an automatic transmission speed conversion is carried out in the optical transmitter and the optical receiver. In particular, when the line switching is required as a result of the fault in the line, the fault recovery time is increased due to the time delay. As the number of the nodes is increased, the transmission time required for transmitting the signal from a transmission side to a receiving side will be further delayed. Accordingly, there is a need for reducing time delays.